WASHINGTON -- A new poll released Thursday by CNN finds Donald Trump close to taking the lead in the GOP primary. But the poll also shows that among a broader swath of respondents, Trump is just plain unpopular.

The CNN survey found that 64 percent of adults said they had an unfavorable opinion of Trump, while only 31 percent have a favorable opinion. Even among Republicans, Trump's ratings in the poll are at best lukewarm: 50 percent had a favorable opinion of Trump while 45 percent had an unfavorable opinion. Still, Trump's tepid support among Republicans was enough to put him in second place in the poll for the GOP primary, where a frontrunner has yet to emerge. Trump's 14 percent on the primary question placed him behind leader Mike Huckabee at 16 percent but ahead of Mitt Romney at 13 percent.

The poll was conducted April 29-May 1 among 1,034 adults, and had a margin of error of three percentage points. The Republican primary question was asked of 475 Republicans and had a margin of error of 4.5 percentage points.

A Rasmussen Reports poll also released Thursday suggests that at least some of Trump's unpopularity could be related to recent statements that questioned President Obama's birth certificate. That poll, conducted May 3-4, found Trump's "unfavorable" rating increased to 66 percent from 53 percent in a survey conducted in April.

As for the birth certificate, a new Washington Post poll found that the percentage of Americans who say Obama was born in the United States has increased to 86 percent. That's up from 77 percent who believed the president was U.S.-born in an April poll concluded just two days before he released his long-form birth certificate. In the same polls, belief that Obama was born in another country dropped from 20 percent to only 10 percent, leaving little traction for the issue Trump has been so vocal about.

On the other hand, a poll conducted by the Democratic firm Public Policy Polling in mid-February -- before Trump ever made any statements about Obama's birth certificate -- found that 56 percent of respondents already had an unfavorable opinion of Trump, calling into question whether his current unpopularity is in fact related to those statements.

Regardless of the origins of his unpopularity, other signs point to difficulty for Trump should he survive the Republican primary. A Quinnipiac University poll released Wednesday found that 58 percent of respondents said they would "never" vote for Trump. The only other potential candidate to receive a majority "never" vote was Sarah Palin.

In addition, the Rasmussen Reports poll found that only 15 percent of respondents think Trump is making a serious run for president and 61 percent think he's just seeking publicity. Similarly a CBS News/New York Times poll from mid-April found that only 23 percent of registered voters (including 37 percent of Republicans) considered Trump a serious candidate, while 72 percent said he was not.